AT89C5132
The bus communication uses a special protocol (MultiMedia Card bus protocol) which is applica-
ble for all devices. Therefore, the payload data transfer between the host and the cards can be
bidirectional.
16.2.1
Bus Lines
The MultiMedia Card bus architecture requires all cards to be connected to the same set of
lines. No card has an individual connection to the host or other devices, which reduces the con-
nection costs of the MultiMedia Card system.
The bus lines can be divided into three groups:
•
•
•
Power supply: VSS1 and VSS2, VDD – used to supply the cards.
Data transfer: MCMD, MDAT – used for bidirectional communication.
Clock: MCLK – used to synchronize data transfer across the bus.
16.2.2
Bus Protocol
After a Power-on reset, the host must initialize the cards by a special message-based MultiMe-
dia Card bus protocol. Each message is represented by one of the following tokens:
•
Command: a command is a token which starts an operation. A command is transferred
serially from the host to the card on the MCMD line.
•
Response: a response is a token which is sent from an addressed card (or all connected
cards) to the host as an answer to a previously received command. It is transferred serially
on the MCMD line.
•
Data: data can be transferred from the card to the host or vice-versa. Data is transferred
serially on the MDAT line.
Card addressing is implemented using a session address assigned during the initialization
phase, by the bus controller to all currently connected cards. Individual cards are identified by
their CID number. This method requires that every card will have an unique CID number. To
ensure uniqueness of CIDs the CID register contains 24 Bits (MID and OID fields) which are
defined by the MMCA. Every card manufacturers is required to apply for an unique MID (and
optionally OID) number.
MultiMedia Card bus data transfers are composed of these tokens. One data transfer is a bus
operation. There are different types of operations. Addressed operations always contain a com-
mand and a response token. In addition, some operations have data token, the others transfer
their information directly within the command or response structure. In this case no data token is
present in an operation. The Bits on the MDAT and the MCMD lines are transferred synchronous
to the host clock.
Two types of data transfer commands are defined:
•
Sequential commands: These commands initiate a continuous data stream, they are
terminated only when a stop command follows on the MCMD line. This mode reduces the
command overhead to an absolute minimum.
•
Block-oriented commands: These commands send data block succeeded by CRC Bits. Both
read and write operations allow either single or multiple block transmission. A multiple block
transmission is terminated when a stop command follows on the MCMD line similarly to the
stream read.
Figure 16-1 to Figure 16-5 show the different types of operations, on these figures, grayed
tokens are from host to card(s) while white tokens are from card(s) to host.
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4173E–USB–09/07
ATMEL [ ATMEL ]
